Brief Report: Alternative Splicing of Extra Domain A (EIIIA) of Fibronectin Plays a Tissue-Specific Role in Hematopoietic Homeostasis

Abstract

Fibronectin (FN) is a major extracellular matrix protein implicated in cell adhesion and differentiation in the bone marrow (BM) environment. Alternative splicing of FN gene results in the generation of protein variants containing an additional EIIIA domain that sustains cell proliferation or differentiation during physiological or pathological tissue remodeling. To date its expression and role in adult hematopoiesis has not been explored. In our research, we demonstrate that during physiological hematopoiesis a small fraction of BM derived FN contains the EIIIA domain and that mice constitutively including (EIIIA 1/1 ) or excluding (EIIIA 2/2 ) the EIIIA exon present comparable levels of hematopoietic stem cells, myeloid and lymphoid progenitors within BM. Moreover, only minor alterations were detected in blood parameters and in hematopoietic frequencies of BM granulocytes/monocytes and B cells. As opposed to other tissues, unique compensatory mechanisms, such as increased FN accumulation and variable expression of the EIIIA receptors, Toll like receptor-4 and alpha9 integrin subunit, characterized the BM of these mice. Our data demonstrate that FN is a fundamental component of the hematopoietic tissue and that the EIIIA exon may play a key role in modulating hematopiesis in conditions of BM stress or diseases. STEM CELLS 2016;34:2263-2268 SIGNIFICANCE STATEMENTFibronectin represents a major extracellular matrix protein implicated in cell adhesion in the bone marrow environment. Alternative splicing of fibronectin gene results in the generation of protein variants containing an additional EIIIA domain that sustains cell proliferation or differentiation during physiological or pathological tissue remodeling. However, its expression and role in adult hematopoiesis has not been explored. Here, we demonstrate that during physiological hematopoiesis a small fraction of fibronectin contains the EIIIA domain and that mice constitutively including or excluding the EIIIA exon present regular hematopoiesis.

Fibronectin and Its Receptors in Hematopoiesis

“…The study of fibronectin's role in hematopoiesis has been fraught with obstacles, and evaluating the role of fibronectin in the hematopoietic stem cells, lymphopoiesis, and erythropoiesis in vivo is still missing. This is partly due to the large number of cells that produce fibronectin or its isoforms, or compensation in the amount of fibronectin or in the expression of fibronectin receptors with various manipulations [96]. Furthermore, circulating fibronectin can infiltrate various tissues and modulate the local production of fibronectin [9].…”

EDA fibronectin–TLR4 axis sustains megakaryocyte expansion and inflammation in bone marrow fibrosis

“…β2microglobulin gene expressions were used for the comparative concentration analysis. PCR splicing assays for human and mouse EDA, mouse EDB, and V region were performed as previously described (Malara et al, 2011(Malara et al, , 2016.…”

“…EDA containing FN has been shown to activate TLR4 in the innate immune response (Okamura et al, 2001). Recently, we demonstrated that mice with constitutive inclusion of EDA exon (EIIIA +/+ ) or knockout for EDA exon (EIIIA −/− ) display regular hematopoietic homeostasis, although tissue-specific compensations in the amount of FN and in the expression of FN receptors were detected (Malara et al, 2016). Despite this knowledge, to date, expression and function of cFN isoforms in bone marrow (BM) fibrosis have not been explored.…”

Alternative Splicing in the Hippo Pathway—Implications for Disease and Potential Therapeutic Targets

“…In contrast, in Drosophila melanogaster 60% of genes are alternatively spliced [ 11 ]. AS is involved in regulating a variety of biological processes including cell division and apoptosis [ 12 ], differentiation during development and in adult tissues [ 3 ] as well as how cells respond to external stimuli (e.g., [ 13 ]). It is therefore not surprising that AS has been implicated in regulating key signalling pathways such as Ras-MAPK and PI3K-mTOR signalling [ 14 ].…”